The present invention relates generally to circuits and techniques for measurement of capacitance, and more particularly to improvements in circuits and techniques adapted for processing digital signals generated by the touch screen controller circuitry that scans a touch screen panel.
Touch screen controller circuits for use in touch screen applications have generally included digital controller circuitry and analog circuitry for detecting/measuring the presence of capacitance if a user touches a point on a touch screen. The presence or movement of a user's finger in the vicinity of the electric field associated with the cross-coupling capacitance in the touch screen disturbs or impedes the electric field and therefore modifies the cross-coupling capacitance. The capacitance detecting/measuring circuit therefore indicates the presence of the finger as a change in the modified touchscreen cross-coupling capacitance. The prior art typically utilizes current sourcing/sinking circuitry, RC networks, and counters to provide a digital indication of the measured capacitance, which, in a touch screen controller, can be used to precisely identify/indicate the screen location being touched.
FIG. 1A illustrates part of a touch screen panel 1-1 which includes a suitable number of horizontal transparent conductors 2 disposed on one surface of a thin, transparent insulative layer (not shown). A suitable number of vertical transparent conductors 3 are disposed on the other surface of the insulative layer. The left end of each of the horizontal conductors 2 can be connected to suitable current sourcing or drive circuitry. The bottom end of each of the vertical conductors 3 can be connected to suitable current sinking or receiving circuitry. A cross-coupling capacitance CSENij occurs at an “intersection” of each horizontal conductor such as 2i and each vertical conductor such as 3j, the intersection being located directly beneath a “touch point” 13. Note that the touching by a user's finger does not necessarily have to occur directly over a touch point. If multiple touch points 13 are sufficiently close together, then a single touching may disrupt the electric fields of a number of different cross-coupling capacitances CSENij. However, the largest change in the value of a particular cross-coupling capacitance CSENij occurs when the touching occurred directly over that particular cross-coupling capacitance.
FIG. 1B illustrates any particular horizontal conductor 2i and any particular vertical (as in FIG. 1A) conductor 3j and the associated cross-coupling capacitance CSENij between them, i and j being row and column index numbers of the horizontal conductors 2 and the vertical conductors 3, respectively. (By way of definition, the structure including the overlapping conductors 2i and 3j which result in the cross-coupling capacitance CSENij is referred to as “capacitor CSENij”. Thus, the term “CSENij” is used to refer both to the capacitor and its capacitance.)
The drive circuitry for horizontal conductor 2i can include a drive buffer 12 which receives appropriate pulse signals on its input 4. The output of drive buffer 12 is connected to the right end of conductor 2i, which is modeled as a series of distributed resistances RA and distributed capacitances CA each connected between ground and a node between two adjacent distributed resistances RA. The receive circuitry for conductor 3j is illustrated as being connected to the right end of vertical conductor 3j. A switch S1j is connected between conductor 3j and VSS. A sampling capacitor CSAMPLE has one terminal connected to conductor 3j and another terminal connected by conductor 5 to an input of a comparator 6, one terminal of a switch S2j, and one terminal of a resistor RSLOPE. The other terminal of switch S2j is connected to VSS. The other terminal of resistor RSLOPE is connected to the output of a slope drive amplifier 9, the input of which receives a signal SLOPE DRIVE. The other input of comparator 6 is connected to VSS. The output of comparator 6 is connected to an input of a “timer capture register” 7, which can be a counter that, together with resistor RSLOPE and capacitor CSAMPLE, perform the function of generating a digital output signal on bus 14 representing the value of CSENij.
A typical prior art touch screen controller circuit uses a methodology of driving a row of a touch screen with a square wave signal, amplifies and filters a resulting column signal, and then digitizes that column signal by means of a delta-sigma ADC or a low resolution pipeline ADC. The digitized column signal is provided as an input to a microprocessor, which executes a program to determine the location of the touched cross-coupling capacitance of the panel. Presently available touch panel products are believed to contain a microprocessor, such as an ARM microprocessor, for processing search digital column signals using complicated algorithms in order to accomplish the detection of the location of a finger touch on the panel.
A problem of the above described prior art is that the time required for the capacitance measurement is time-varying in the sense that a lower value of the capacitance CSENij requires less counting time by timer capture register 7, whereas a higher value of the capacitance CSENij requires more counting time by timer capture register 7. The widely variable capacitance measurement times may be inconvenient for a user. Also, the system is quite susceptible to noise because comparator 6 in Prior Art FIG. 1B is connected via CSAMPLE during the entire capacitance measurement process.
Another problem of the above described prior art is that it is unsuitable for applications in which a small, inexpensive, low power touch screen controller circuit is needed because of the complexity of the above described prior art.
Thus, there is an unmet need for a capacitance measurement system that is capable of making accurate measurements of a broader range of capacitances than the prior art.
There also is an unmet need for an improved digital circuit and method for making touch screen capacitance measurements in a touchscreen controller circuit.
There also is an unmet need for a digital capacitance measurement system and method having greater capacitance measurement sensitivity than the prior art.
There also is an unmet need for a digital capacitance measurement system and method having greater capacitance per LSB measurement sensitivity than the prior art.
There also is an unmet need for a digital capacitance measurement system and method having a programmable ability to actively compensate for large parasitic capacitances.
There also is an unmet need for a digital capacitance measurement system and method that is more suitable than the prior art for applications in which a small, inexpensive, low power touch screen controller circuit is needed because of the complexity and size of the prior art.
There also is an unmet need for a digital capacitance measurement system and method that allows simultaneous driving of multiple rows of a touch screen panel without overdriving or saturating an amplifier which receives column signals generated as a result of the simultaneous driving of multiple rows.